1. Laboratório de Novos Materiais Isolantes e Semicondutores (LNMIS), Institute of Physics, Federal University of Uberlândia, CP 593, Uberlândia MG 38400-902, Brazil
2. Center for Applied Physics, Institute of Physics, University of Brasília, Brasília DF 70910-900, Brazil
3. School of Automation, Huazhong University of Science and Technology, Wuhan 430074, China